Several species of nearly linear dye molecules incorporated into the nerve were found to undergo rotational motion within the membrane and to assume orientations nearly perpendicular to the membrane surface when the nerve was excited electrically. This was shown by an increase in absorption at the edge of an axon, of light polarized in the direction perpendicular to the membrane surface; simultaneously, there was a decrease in the absorption of light polarized in the direction parallel to the surface. By using intracellularly perfused squid giant axons, competition between calcium and alkali metal ions at the negatively charged sites of the nerve membrane was examined. Experimental evidence was obtained indicating the cooperative conformational transitions of the membrane macromolecules associated with giant fluctuations near the critical points for transitions.